Presence servers are increasingly being used to provide presence information such as the availability status of users. Presence information of a user identifies the current “presence state” of that user. Users may make their presence information available so that other users can decide how best to communicate with them. For example, the presence information may indicate whether a user is logged on (“online”) with an instant messaging server or is logged off (“offline”). Presence information may also provide more detailed information about the availability of the user. For example, even though a user is online, that user may be away from their computer in a meeting. In such a case, the presence state may indicate “online” and “in a meeting.”
In an instant messaging context, a publishing user (“publisher”) may provide their presence information to a presence service that then provides the presence information to subscribing users (“subscribers”). Thus, a presence service may use a subscriber/publisher model to provide the presence information for the publishing and subscribing users of the presence service. Whenever the presence information of a user changes, the presence service is notified of the change by that user's computer system and in turn notifies the subscribing users of the change. A subscribing user can then decide how best to contact the publishing user based on the presence information of the intended participants. For example, if the presence information indicates that a publishing user is currently in a conference telephone call, then the subscribing user may decide to send an instant message, rather than place a telephone call, to the publishing user. If the subscribing user, however, needs to call and speak with the publishing user, the subscribing user can monitor the presence information of the publishing user to know when the call can be placed. When the subscribing user notices that the publishing user's presence information indicates that the telephone conference has been concluded, the subscribing user can then place the telephone call.
A particular user may also publish presence information from multiple devices. For example, a user may be logged on to a presence service from a laptop computer, a desktop computer, and a cell phone all at the same time. The presence information may indicate the user's status on each device. This is particularly useful if, for example, the user is walking to a meeting with a cell phone and has stepped away from their desktop computer. If a subscribing user wants to contact the publishing user, the presence information may indicate that a call to the cell phone would be a more effective way to reach the user than an instant message received at the user's desktop computer. Each of a user's devices may also subscribe to the presence information of the user's other devices to enable the devices, for example, to determine which device the publishing user is currently most active at.
Because of the increasing popularity of instant messaging systems and other real-time communications systems, presence services need to support an increasing number of users. In addition, these systems need to support increasingly complex definitions of “presence information.” For example, presence information may include calendar information that specifies the availability of a publisher during the next several months. As a result, a presence server is typically developed to provide presence services in an efficient way. Unfortunately, typical presence models require intense computation when presence information is updated. Thus, as more publishers and subscribers are added, additional presence servers are needed to perform the necessary computations.
Early presence systems handled the publishing of presence information by placing all of the presence information in a single XML document. As parts of the user's presence information changed, a publisher had to point to a line in the document and replace it with new presence information or point to a node within an XML subtree and replace it with another XML subtree. This kind of update required the presence server to perform computationally expensive XML parsing. As the number of users and presence updates increase, the burden of XML parsing on the presence server could become overwhelming. This model of publishing presence information also does not allow a subscriber to subscribe to only a portion of a presence document. In addition, individual portions of the document are not versioned such that a subscriber or presence server can easily tell which portions of the document have recently changed. This model also does not allow exposing different versions of a presence document to different subscribers or specifying different authorization information for individual portions of the document.
A more efficient presence system that is less computationally expensive is described in a related application, U.S. patent application Ser. No. 11/190,503 entitled “ORGANIZING PRESENCE INFORMATION INTO COLLECTIONS OF PUBLICATIONS,” filed on Jul. 26, 2005, which is incorporated herein in its entirety by reference. The related application describes a method and system for managing presence information based on collections of publications. A presence collection system creates collections of publications for a publisher. A publication includes a type and value, and may be accompanied by attributes such as when the publication should expire. For example, a publication with a type “status” and a value “online” may indicate to subscribing users that the publishing user is currently online. A publisher may define one collection of publications that is to be made available to certain subscribers and another collection of publications that is to be made available to other subscribers. For example, one collection may be visible to the public, while another may be visible only to coworkers of the publishing user. Each collection contains a list of the users that are allowed to subscribe to the information within that collection. The presence collection system may allow a publisher to update individual publications within a collection. Upon receiving the update, the presence server can quickly update the presence information without having to rely on computationally expensive parsing of XML documents. Each collection of publications may also contain categories of information. A category is a type of publication that spans multiple collections. For example, a category “status” may have a publication with a value “not available” in a collection that is available to the public, and a publication with a value “in a meeting with John” in a collection available to coworkers. Publications may also specify an instance identifier. An instance identifier differentiates similar information within the same collection. For example, a user may have a publication “status” with an instance of “laptop” that indicates the status of the user at their laptop, and another publication “status” with an instance of “cell phone” that indicates the status of the user at their cell phone.
Although the presence collection system is an improvement over prior techniques, it may place a higher burden on network resources and the presence server. For example, a publishing user who sends separate updates to many publication instances within many collections to a presence server creates a high amount of network traffic to the presence server. Similarly, a subscribing user who receives separate notifications for each publication that is updated creates a high amount of network traffic from the presence server to the subscribing user's logged-on device or devices. Typically, presence publications are set to expire within a fixed amount of time, and must be periodically refreshed by the publishing user in order to keep the publications published. The need to refresh many publications that have not changed creates an unnecessary burden on the server. Finally, the addition of collections having varying degrees of detail available to different subscribing users creates an opportunity to expose presence information to broader groups of subscribers. The list of users with access to a collection can grow quite large and the computational cost of checking which collection a particular subscriber should be subscribed to only increases as the list grows, causing an additional burden to the presence server.